When you're doing things secret and sneakily, you're doing them surreptitiously.
This is an adverb that applies to actions that you're trying to do covertly: you don't want anyone to know about them, so you have to be sneaky. Burglars approach a house surreptitiously. If you plan a surprise party for your mother, you have to go about it surreptitiously or she'll find out and the surprise will be ruined. Anything you don't reveal fully or do on the sly is an example of going about it surreptitiously.